Coupled cavity slow wave circuit for microwave tubes

ABSTRACT

A coupled cavity slow wave circuit and microwave tube using same are disclosed. The coupled cavity slow wave circuit includes an array of cavity resonators coupled together via the intermediary of an array of slots communicating through the common walls of adjacent resonators. The slots are dimensioned to have a resonance frequency substantially at the upper band edge frequency of the cavity passband mode such that the higher frequency passband of the slot mode is coalesced with the lower passband of the cavity mode. The coupling slots are arranged in an array such that the centers of the coupling slots in opposite end walls of the coupled cavities are aligned in axial direction of the slow wave circuit along a line which is generally parallel to the axis of the same beam. This in-line slot configuration allows higher interaction impedance in the 1.5 pi to 1.8 pi radians of phase shift per cavity period without band edge instability.

United States Patent James et al.

COUPLED CAVITY SLOW WAVE CIRCUIT FOR MICROWAVE TUBES [451 Aug. 15, 1972 OTHER PUBLICATIONS Power Travelling Wave Tubes by Gittins, Copy- [72] Inventors: Bertram G. James, Redwood City; right 1965 pages 77 Ward A. Harman Los Altos Hills Primary Examiner-Herman Karl Saalbach m. Rueu Los Altos of Assistant Examiner-Saxfield Chatmon, Jr.

Attorney-Stanley Z. Cole and Leon F. Herbert [73] Assignee: Varian Associates, Palo Alto, Calif. [22] Filed: Sept. 3, 1970 E5 1 d l d coupe cavity sow wave circuit an microwave [21] Appl' 69l98 tube using same are disclosed. The coupled cavity slow wave circuit includes an array of cavity resona- 52 us. Cl ..31s/3.s, 333/31 tors coupled together via the intermediary of an array 511 lm. Cl .3011 25/34 of slots communicating through the m on walls of [58] Field of Search ..31s/3.5, 3.6, 39.3; 330/42; adlacem resonawrs- The are dlmensloned 333/31 have a resonance frequency substantially at the upper band edge frequency of the cavity passband mode such that the higher frequency passband of the slot [56] Reerences CM mode is coalesced with the lower passband of the cavi- UNITED STATES NT ty mode. The coupling slots are arranged in an array such that the centers of the coupling slots m opposite 3,297,906 l/ 1967 Schumacher ..3l5/3.5 end walls f the coupled cavities are aligned in axial 3,221,204 11/1965 Hant et al. ..315/3.5 direction f the slow wave circuit aiong a line which is Mayel'hofer generally to the axis of the Same beam Allen et al. in-line slot configuration allows higher interaction im- 3,471,738 10/1969 Bert ..333/3l X da in the 1.51; to 1.81r radians of phase shift per cavity period without band edge instability.

6 Claim, 8 Drawing Figures I I 12h 2| 2 L/ /l/ U /l 1 l cAvm/l l SLOT l PATENTEDAUB 15 ran SHEET 1 BF 3 CAVITY S FR QUENCY FR ENY 5%? w 1 '2 1i LUZ 9 9 I i mfi h ""T PULSER 1 5 L 6 u f POWER OUTPUT (KW) FIG. 8

' 3 l2 Ffifi n M M GAIN =54.7dB w o 9E12-' 9,". 2 l0- x1 IRE Q RENL SHORTING FIG. 3 6- PLANES INVENTORS BERTRAM 6. JAMES WARD A. vHARM/\N JOHN A. RUETZ -224 -||'2 Isis +||'2 +2'24 BY Q MHz MHz CH2 MHZ MHz FREQUENCY NEY COUPLED CAVITY SLOW WAVE CIRCUIT FOR MICROWAVE TUBES DESCRIPTION OF THE PRIOR ART I-Ieretofore, in coupled cavity slow wave circuits, the higher frequency slot mode and the lower frequency cavity mode have been coalesced at the upper band edge of the cavity mode to substantially increase the cold bandwidth of the composite slow wave circuit. However, in this prior coupled cavity circuit, the coupling slots were staggered in successive end walls of the cavities such that the waves traveling through the slow wave circuit meandered across the interaction gaps in the fashion of a folded waveguide. The problem with this geometry is that although the cold bandwidth of the circuit is substantially increased due to the coalescing of the slot and cavity modes, the backward wave branch of the (-5 diagram has substantial interaction impedance at the Zn phase shift per period portion of the circuit and the circuit tends to break into backward wave oscillation at the 271 point of the cavity mode. Thus, although coalescing the slot and cavity modes, in the staggered coupled cavities circuit, increases substantially the cold bandwidth of the circuit the usable bandwidth of the circuit for a hot tube is substantially less than the cold bandwidth of the circuit. Therefore, the prior art staggered coalesced slot and cavity mode coupled cavity circuit has not been used to any substantial extent in microwave tubes.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved coupled cavity slow wave circuit and microwave tubes utilizing the same.

One feature of the present invention is the provision in a coupled cavity slow wave circuit for a microwave tube, of an array of coupling slots each having a resonant frequency at substantially the upper band edge frequency of the cavity mode to coalesce the high frequency band edge of the cavity mode and the low frequency band edge of the slot mode for increasing the bandwidth of the passband of the composite slow wave circuit, and wherein coupling slots in opposite end walls of each of the coupled cavities have their geometric centers lying on the same side of the beam path and falling substantially in a plane defined by the axis of the beam path and the centers of the coupling slots, whereby the interaction impedance of the gap at the 27! phase shift per cavity operating point is substantially negligible to prevent backward wave oscillation of the tube.

Another feature of the present invention is the same as any one or more of the preceding features wherein the beam is cumulatively electromagnetically interacted with the wave energy in the circuitjn the region of 1.01r to 2.011- radians of phase shift per cavity of the coupledcavity slow wave circuit, whereby the interaction impedance of the circuit is substantially increased.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram, partly in block diagram form, partly in line diagram form, and partly in sectional view, depicting the prior art microwave tube,

FIG. 2 is a perspective schematic line diagram of one of the coupled cavities of the prior art slow wave circuit of FIG. 1,

FIG. 3 is a schematic enlarged cross-sectional view of an alternative embodiment of that portion of the struc ture of FIG. 1 delineated by line 3-3 and employing features of the present invention,

FIG. 4 is a schematic perspective line diagram depicting one of the coupled cavities of the circuit of FIG.

FIG. 5 is an 10-13 diagram depicting the passbands for the cavity mode, slot mode, and coalesced cavity and slotmode circuits,

FIG. 6 is a plot of normalized frequency versus normalized phase shift per cavity depicting the dispersive characteristics of the staggered coupled cavity circuit of the prior art and for the in-line coalesced mode slow wave circuits of the present invention,

FIG. 7 is a plot of interaction gap impedance versus normalized phase shift per cavity depicting the characteristics for the coalesced mode circuit of the present invention and for the staggered and staggered coalesced mode circuits of the prior art, and

FIG. 8 is a plot of power output in kilowatts versus frequency in GHZ depicting the output characteristics of a traveling wave tube employing. features of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown a prior art microwave tube. The prior art tube 1 includes an electron gun 2 disposed to project a beam of electrons 3 over an elongated beam path to a collector electrode 4 for collecting and dissipating the energy of the beam. A coupled cavity slow wave'circuit 5 is disposed along the beam path 3 for cumulative electromagnetic interaction with the beam. In one embodiment of the prior art coupled cavity slow wave circuit 5 the circuit is arranged for operation in the phase shift per cavity portion of the w-B diagram such that between 1.011 and 2.01r phase shift occurs between successive interaction gaps of the circuit. In other words, the circuit is operated in the second spatial harmonic of the (0-3 diagram for amplification of wave energy coupled onto the circuit 5 via an input coupling means such as an input coupling iris 6. Wave energy cumulatively interacts with the beam to produce an amplified output wave which is coupled from the downstream end of the circuit via an output iris 7 to a waveguide 8 communicating with a load, such as an antenna, not shown.

The prior art coupled cavity slow wave circuit includes a longitudinal array of cavity resonators 9 coupled together via the intermediary of an array of coupling slots 11, which are staggered along the circuit in opposite end walls of each of the cavities. More specifically, coupling slots 11 alternate from one side of the beam to the other as the wave energy propagates through the circuit in the mean direction of the beam. This will hereinafter be referred to as a staggered" coupled cavity circuit, as shown in FIG. 2.

The prior art staggered coupled cavity slow wave circuit 5 has an (-3 diagram as shown in FIG. by curves 12 and 13. Curve 12 represents the -13 diagram for the first or low frequency passband of the cavity mode and curve 13 of the (0-3 diagram represents the higher passband or the slot mode of the staggered coupled cavity circuit 5. This prior art circuit has been coalesced, in the prior art, by properly dimensioning the cavities 9 and by tuning the resonant frequency of the slots 11 to a frequency substantially below the upper passband edge of the cavity mode. When the two modes, namely, the slot mode and the cavity mode are coalesced, for the staggered slot configuration circuit, the resultant coalesced mode circuit has an 00-3 diagram as shown by curves 14 and 15 of FIG. 5. Thus, the cold bandwidth for the coalesced mode staggered slot circuit is substantially increased as compared with the circuit before the modes are coalesced. However, as seen by reference to FIG. 7, the interaction impedance of the uncoalesced staggered slot circuit increases to very high impedances for phase shifts per cavity between 1.61r and 2.01r radians, whereas the impedance for the coalesced mode staggered slot circuit levels out at a relatively high value.. p

This means that, for the prior art staggered slot circuits whether coalesced or not, the beam velocity must be arranged for interaction generally below 1.51r radians of phase shift per cavity or else interaction is obtained due to the high interaction impedance, near the 2.01r radians of phase shift per cavity, causing the tube to enter into oscillation.

Therefore, careful turn-on procedures must be employed for turning on the beam such that the beam voltage does not pass through a lower beam voltage than that required for operation between 1.0'rr and 1.51r radians of phase shift per cavity or else the circuit breaks into oscillation.

Another disadvantage to operation in the phase shift per cavity region between 1.01r and 1.51r radians per cavity is that at relatively high frequencies, such as at millimeter wavelengths, the cavity length or common wall thickness between cavities for a fixed beam velocity gets relatively small. Thus, the cavity length of the individual cavity gets relatively small, thereby decreasing the interaction impedance or if the length of the cavity is maintained constant, the common wall thickness between the adjacent cavity is reduced, thereby decreasing the thermal capacity of the circuit.

Referring now to FIGS. 3 and 4, there is shown the coupled cavity slow wave circuit 19 of the present invention. Coupled cavity slow wave circuit 19 is substantially the same as that of FIG. 1 with the exception that the coupling slots 21 are arranged with their geometric centers in substantial axial alignment with a line which is parallel to the beam axisand disposed to one side of the beam axis 3. In addition, the coupling slots 21 are dimensioned to have a resonance frequency substantially at the upper band edge frequency of the cavity mode, as indicated by line 12 of FIG. 5. In this manner, the in-line coupled cavity slow wave circuit is coalesced to provide coalesced w-B curves, as shown by curves 14 and 15 of FIG. 5.

The coalesced mode in-line slot circuit has a normalized frequency versus phase shift characteristic as indicated by the family of curves identified by numeral 22 in FIG. 6. In addition, the coalesced mode in-line circuit 19 of FIG. 3 and 4 has an interaction impedance versus phase shift per cavity identified by the family of curves 24 of FIG. 7. This family of curves is to be distinguished from the family of curves for the coalesced staggered circuit of FIGS. 1 and 2, inasmuch as the interaction gap impedance of the coalesced in-line circuit tends to substantially decrease near the 211' radians of phase shift per cavity operating point, whereas the prior art staggered slot circuit has the interaction gap impedance for the uncoalesced case approaching infinity at a 211' operating point or for the coalesced case leveling out at a relatively high value at the 21r point.

Thus, in the in-line coalesced circuit mode of FIGS. 3 and 4, the beam voltage may be swept through the Zn operating point without encountering instability in the tube and without causing backward wave oscillations. Thus, the circuit of FIGS. 3 and 4 may be operated in the phase shift per cavity region between 1.511 and 1.81:- radians without encountering instability. This allows the period of the circuit to be substantially increased for a given operating frequency, thereby permitting higher interaction gap impedance for the circuit, alternatively by trading off wall thickness for interaction gap impedance, the thermal capacity of the circuit can be substantially increased over the prior art staggered circuit of FIGS. 1 and 2.

More particularly, the coalesced in-line circuit of FIGS. 3 and 4 can be made to operate at a normalized phase shift per cavity between 1.61-r and 1.811 radians per cavity. This represents approximately a 25 percent greater period length for the coalesced in-line circuit as compared with the coalesced mode staggered circuit. This extra length can either be put into cavity height for increasing the interaction impedance or it can be used to increase the wall thickness between cavities 9. For example, with an initial wall thickness representing 25 percent of the period and a cavity height of percent, a 25 percent increase in period permits the same cavity height to be used with double the wall thickness between cavities 9. This essentially doubles the power handling capability of the circuit.

An additional important feature of the coalesced inline circuit 19 is its immunity to band edge instabilities. As previously described, slow wave circuits having a more usual phase characteristic, such as curves 25 and 26 of FIG. 7 and curves 27 and 28 of FIG. 6, the slope of the phase diagram approaches zero at the band edges and causes high interaction impedance at these frequencies. Since the circuit matches are also generally poor at these frequencies, resonant monotron-type oscillations are frequently encountered unless special design techniques are employed to suppress these oscillations. Again, due to the unique properties of the coalesced in-line circuit 19, the cavity mode, which produces the interaction fields in the cavity gaps, is not supported at the 21r point in the circuit. This is due principally to the fact that in the coalesced in-line structure, the slots are resonant at this frequency, whereas the slot resonant frequency in a coalesced staggered circuit, of the prior art, is always below this point in frequency. The cavity gap mode does not receive appreciable excitation in the in-line coalesced mode circuit under conditions where the slot represents a high impedance. In a way this is analogous to trying to excite a half wave section of open circuit transmission line by producing a voltage maximum at the center rather than-at the ends of the line. The almost complete suppression of the gap interaction impedance associated with the in-line circuit at the 211* point has been observed in operating tubes.

The total gap interaction impedance V /2P of a couf pled cavity circuit is determined principally by the cavity R/Q and the slope of the phase curve. The interaction impedance is always relatively high in regions where the circuit group velocity is low. The effective beam interaction impedance also depends upon the gap coupling coefficient so that a figure of merit for coupled cavity circuits becomes (R/Q)(M/Bp) FIG. 7 shows the total gap interaction impedance of the family of circuits presented in FIG. 6. All circuits have been calculated for R/Q 50. The impedances obtained with the staggered circuit configuration of FIGS. 1 and 2 are contained between the limits of curves (A) and (B). The coalesced inline circuit tends to produce higher impedance at low values of Bp because of the low group velocity associated with the circuit under these conditions.

Measured impedance data has been found to agree well with calculated equivalent circuit data except for predicting the zero interaction impedance which is experimentally observed at the Zn point in the case of the coalesced in-line circuit.

In a microwave tube of the type shown in FIGS. 3 and 4, constructed for operation at Ku-band, the phase curve corresponded to the curve that has an L/C ratio of 1,000 in FIGS. 6 and 7. This tube was operated at a perveance of 0.8 X and the beam voltage was 20.7 kilovolts at a beam current of 2.35 amps. FIG. 8 shows a plot of the power output as a function of the frequency. The maximum power measured was 8.4 kilowatts cw with a 1 db bandwidth of 2 percent and a saturated gain of 34.7 db. This tube was constructed with no beam current modulating electrode. Also no wave attenuative material was added to the circuit. Measured insertion loss of the circuit section was approximately 1 db. Beam power was applied to the tube by varying the beam voltage from zero to the operating voltage. The beam voltage was varied through the 211 point to search for band edge instabilities. No instabilities were observed. The elimination of the need for additional circuit loss greatly simplified the fabrication techniques, particularly at the extremely small circuit size associated with millimeter waves.

Although the coupled cavity in-line circuit of FIGS. 3 and 5 has been described as used for forward wave interaction in a traveling wave tube operating with a phase shift per period between 1.511- and 1.81; radians per cavity, it may also be used to advantage as the output circuit for a hybrid tube utilizing a succession of klyst'ron buncher cavities followed by the in-line coupled cavity circuit of FIGS. 3 and 4 as the output circuit.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is: 1. In a microwave tube; means for pro ecting a beam 10 jacent-ones of said cavity resonator having common end walls, an array of wave energy coupling slot means disposed in the common end walls of said cavity resonators for wave energy coupling together the array of cavity resonators to define a slow wave circuit haw ing a cavity mode passband of frequencies associated with the coupled cavities and a slot mode passband of frequencies centered at a higher frequency than the cavity mode passband and associated with the array of slots, the improvement wherein, said slots in opposite end walls of said coupled cavities are disposed with their centers on one side of said beam path and are dimensioned relative to the dimensions of said cavity resonator to have a resonant frequency at substantially the upper band edge frequency of the cavity mode to coalesce the high frequency band edge of the cavity mode and the low frequency band edge of the slot mode for increasing the width of the operating passband of the tube employing said slow wave circuit.

2. The apparatus of claim 1 wherein said coupling slots in opposite end walls of each of said cavities have their geometric centers falling substantially in a plane defined by the axis of the beam path and the centers of said coupling slots.

3. The apparatus of claim 1 wherein said coupled cavity slow wave circuit means is a backward wave circuit for the fundamental space harmonic.

4. In a method for stabilized operation of a coupled cavity slow wave tube operating with cumulative electromagnetic interaction with the beam in the region of 1.011 to 2.07:- radians of phase shift per period of the slow wave circuit the step of, dimensioning the coupling slot between adjacent cavity resonators of the circuit to a slot mode of resonance substantially at the upper band edge frequency of the cavity mode passband of the slow wave circuit to coalesce the low frequency passband edge of the slot mode with the upper passband edge of the cavity mode, and positioning the centers of said coupling slots in opposite end walls of each of the coupled cavities to lie on one side of the beam path, thereby substantially increasing the width of the operating passband of the tube employing said composite slow wave circuit and eliminating the stop band of frequencies between the cavity and slot modes of propagation.

5. The method of claim 4 including the step of, positioning the centers of said coupling slots in opposite end walls of each of said coupled cavities to fall substantially in a plane defined by the axis of the beam and the geometric centers of the coupling slots.

6. The method of claim 4 including the step of, positioning the coupling slots in opposite end walls of each of the coupled cavities to lie substantially in a straight line generally parallel to the beam axis. 

1. In a microwave tube; means for projecting a beam of electrons over an elongated beam path; slow wave circuit means disposed along the beam path in electromagnetic energy exchanging relation with the beam; said slow wave circuit means including, an array of cavity resonators arranged along the beam path for successive electromagnetic interaction with the beam, adjacent ones of said cavity resonator having common end walls, an array of wave energy coupling slot means disposed in the common end walls of said cavity resonators for wave energy coupling together the array of cavity resonators to define a slow wave circuit having a cavity mode passband of frequencies associated with the coupled cavities and a slot mode passband of frequencies centered at a higher frequency than the cavity mode passband and associated with the array of slots, the improvement wherein, said slots in opposite end walls of said coupled cavities are disposed with their centers on one side of said beam path and are dimensioned relative to the dimensions of said cavity resonator to have a resonant frequency at substantially the upper band edge frequency of the cavity mode to coalesce the high frequency band edge of the Cavity mode and the low frequency band edge of the slot mode for increasing the width of the operating passband of the tube employing said slow wave circuit.
 2. The apparatus of claim 1 wherein said coupling slots in opposite end walls of each of said cavities have their geometric centers falling substantially in a plane defined by the axis of the beam path and the centers of said coupling slots.
 3. The apparatus of claim 1 wherein said coupled cavity slow wave circuit means is a backward wave circuit for the fundamental space harmonic.
 4. In a method for stabilized operation of a coupled cavity slow wave tube operating with cumulative electromagnetic interaction with the beam in the region of 1.0 pi to 2.0 pi radians of phase shift per period of the slow wave circuit the step of, dimensioning the coupling slot between adjacent cavity resonators of the circuit to a slot mode of resonance substantially at the upper band edge frequency of the cavity mode passband of the slow wave circuit to coalesce the low frequency passband edge of the slot mode with the upper passband edge of the cavity mode, and positioning the centers of said coupling slots in opposite end walls of each of the coupled cavities to lie on one side of the beam path, thereby substantially increasing the width of the operating passband of the tube employing said composite slow wave circuit and eliminating the stop band of frequencies between the cavity and slot modes of propagation.
 5. The method of claim 4 including the step of, positioning the centers of said coupling slots in opposite end walls of each of said coupled cavities to fall substantially in a plane defined by the axis of the beam and the geometric centers of the coupling slots.
 6. The method of claim 4 including the step of, positioning the coupling slots in opposite end walls of each of the coupled cavities to lie substantially in a straight line generally parallel to the beam axis. 